1. Field of the Invention
The present invention is related to the field of telecommunications, and in particular, to estimating the length of a transmission medium (e.g., subscriber loop in a DSL system).
2. Background
DSL communications use copper phone lines (e.g., twisted pair) for high-speed data transmission. A major problem for DSL service providers is to accurately qualify a subscriber's local loop (sometimes referred to as probing the line) prior to the deployment of DSL service. In general, line probing involves measuring line parameters such as loop capacitance and loop resistance. A typical approach for probing the line requires a first handset to be attached to one end of a telephone line at the telephone company's Central Office (CO) location and a second handset to be attached to the other end of the telephone line at the customer premises equipment (CPE) location. Thus, human interaction is required at two points of the telephone line, including a service call to the CPE location, which increases the cost of deployment. Single ended line probing (SELP) techniques eliminate the need for a service call to the CPE location and the additional costs of such service.
Conventional SELP techniques use a resistor-capacitor (RC) circuit model to estimate the length of a transmission medium, as shown in FIG. 1A. The RC circuit model typically includes the known source resistance Rs of the line and the unknown line capacitance C1. The line capacitance is approximately proportional to the length L of the transmission medium. A Direct Current (DC) pulse is applied to the line and the charge-up time tc is monitored. This function is sometimes provided on a digital multimeter. Once the charge-up time is known (and given a value for Rs), the line capacitance C1 can be estimated along with the line length. FIG. 1B is a graph illustrating the voltage across the line capacitance C1 as a function of time.
Unfortunately, such probing technique cannot be employed in a typical CO DSL modem. Conventional DSL modem boards typically include a line transformer to isolate the line from the CO equipment and reject the common mode voltage of the line. Due to the presence of the line transformer, a DC pulse cannot be transmitted onto the line. Additionally, a typical CO DSL modem includes extensive high pass filtering for blocking out low frequency signals, including any low frequency line-probing signals. Furthermore, the resistance of the RC circuit model typically includes the source resistance Rs, but ignores the line resistance. This is a reasonable approximation of the resistance when the source resistance is much larger than the line resistance. In conventional DSL modem boards, however, the output resistance of the source is typically not significantly larger than the line resistance. As such, ignoring the line resistance degrades the accuracy of the loop length estimate.
Accordingly, there is a need for an improved SELP technique that is suitable for use with typical DSL modems and that can provide an accurate estimate of transmission line length.